Mutations affecting mitochondrial biogenesis, respiratory function and other aspects of energy metabolism are employed to elucidate these processes in the eukaryote, Saccharomyces cerevisiae. Particular emphasis is placed on mutants with abnormalities in porphyrin biosynthesis and its regulation. These mutants are used to investigate biogenesis of cytochromes and other hemoproteins. Mutants requiring delta-aminolevulinic acid for cytochrome synthesis are employed to determine the role of porphyrins and lipids in controlling cytochrome biosynthesis. Levulinic acid, an antimetabolite, is used to inhibit hemoprotein synthesis in normal yeast. Mutants resistant to this antimetabolite are being investigated in elucidating the regulation of porphyrin biosynthesis. Induction of porphyrin accumulation induced by metal ions, antibiotics and mutation are also under study. In vitro studies of the first enzyme of the porphyrin biosynthesis pathway, delta-aminolevulinic acid synthetase, are in progress. Enzyme from a temperature sensitive mutant requiring delta-aminolevulinic acid for growth will be tested to determine whether the mutation involves the structural gene for the synthetase. Other studies include a characterization of a mutation which alters both porphyrin metabolism and the storage of carbohydrate energy reserves.